High speed optical phase modulator

ABSTRACT

An optical device includes a phase modulator having a waveguide on a substrate. The phase modulator also includes an n-type region having a proximity to a p-type region that causes formation of a depletion region when a bias is not applied to the modulator. The depletion region is at least partially positioned in the light signal carrying region of the waveguide. The phase modulator is tuned by applying a reverse bias to the phase modulator. The reverse bias changes the size of the depletion region.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/147,403, filed on Jun. 7, 2005, now U.S Pat. No. 7,394,949 entitled“High Speed Optical Intensity Modulator;” U.S. patent application Ser.No. 11/147,403 claims the benefit of U.S. Provisional Patent ApplicationSer. No. 60/605,589, filed on Aug. 30, 2004, entitled “High SpeedSilicon Modulator;” and U.S. patent application Ser. No. 11/147,403 alsoclaims the benefit of U.S. Provisional Patent Application Ser. No.60/577,636, filed on Jun. 7, 2004, entitled “Ultra-Fast SiliconModulators Based on Reverse Bias PN and PIN Junctions,” and thisapplication is a continuation of U.S. patent application Ser. No.11/146,898, filed on Jun. 7, 2005, now U.S. Pat. No. 7,394,948 entitled“High Speed Optical Phase Modulator;” U.S. patent application Ser. No.11/146,898 claims the benefit of U.S. Provisional Patent ApplicationSer. No. 60/605,589, filed on Aug. 30, 2004, entitled “High SpeedSilicon Modulator;” and U.S. patent application Ser. No. 11/146,898 alsoclaims the benefit of U.S. Provisional Patent Application Ser. No.60/577,636, filed on Jun. 7, 2004, entitled “Ultra-Fast SiliconModulators Based on Reverse Bias PN and PIN Junctions,” each of which isincorporated herein in its entirety.

FIELD

The present invention relates to optical devices and more particularlyto optical modulators.

BACKGROUND

Optical modulators are used to encode information onto light signals. Itis desirable to encode information at a rate of about 10 to 40 Gbps.However, encoding data at these rates has proven difficult due to thelimitations of optics and the associated electronics. Existing practiceinvolves making these devices using exotic materials like LiNbO₃ and InPfor the optical modulation and GaAs hetero-junction-bipolar (HT)transistor for the driver circuit, which makes them expensive and notpractical for many applications. As a result, there is a need for apractical modulator that can encode data at these rates.

SUMMARY

An optical device that includes a phase modulator is disclosed. Thephase modulator includes a waveguide on a substrate. The phase modulatoralso includes an n-type region having a proximity to a p-type regionthat causes formation of a depletion region in the waveguide when a biasis not applied to the modulator. The waveguide can be a ridge waveguideand the depletion region can be located in the ridge. In some instances,the n-type region contacts the p-type region.

The phase modulator can include an electrical connection for applying abias signal to the top of the waveguide and also to a contact regionlocated adjacent to the waveguide and spaced apart from the waveguide.In some instances, the connection for applying a bias signal to the topof the waveguide includes a plurality of contacts spaced apart from oneanother on the top of the waveguide such that contacts are notpositioned over the center of the waveguide. In one example, theelectrical contacts include doped polysilicon. The contact region of thephase modulator can include a secondary doped region in contact with then-type region or the p-type region and can also have a higher carrierconcentration than the contacted region.

In some instances, the phase modulator includes a plurality ofsub-modulators that are spaced apart from one another along the lengthof the waveguide. Each of the sub-modulators is connected to atransmission line that carries a bias signal to each of thesub-modulators. The length of the sub-modulators along the waveguide andthe spacing between the sub-modulators is selected such that the averagespeed of the bias signal through the transmission line is about the sameas the average speed of a light signal traveling through the waveguide.

One or more of the phase modulators can be included in an intensitymodulator such as mach-zehnder interferometer. The intensity modulatorcan be used with electronics that apply a different bias signal to eachof the different modulators. The electronics can provide a delay betweenthe application of the bias signal to different modulators. The delaysynchronizes the light signal and the bias signals so each bias signalresults in modulation of the same portion of the light signal.

A method of using the phase modulator is also disclosed. The methodincludes applying a reverse bias to the phase modulator and tuning thereverse bias so as to adjust the size of the depletion region.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1A is a top-view of a portion of an optical device that includes aphase modulator configured to modulate the phase of a light signaltraveling along a waveguide. Dashed lined show the location of a firstconducting member and a second conducting member. The first conductingmember and a second conducting member are treated as transparent toreveal the underlying features.

FIG. 1B is a cross section of the optical device illustrated in FIG. 1Ataken along the line labeled B.

FIG. 1C illustrates the formation of a depletion region formed in thewaveguide of FIG. 1A as a result of the proximity between doped regions.

FIG. 1D illustrates the effect on the depletion region of applying areverse bias applied to the phase modulator.

FIG. 2A illustrates the electrical contacts and a waveguide associatedwith a phase modulator. The relationship between the contacts and thefundamental mode of a light signal traveling through the waveguide isshown.

FIG. 2B and FIG. 2C illustrate alternate arrangements for the electricalcontacts and waveguides of the phase modulator.

FIG. 3A is a cross section of a phase modulator having contact regionspositioned on opposing sides of a ridge waveguide.

FIG. 3B is a cross section of a phase modulator having contactspositioned on the opposing edges of a channel waveguide.

FIG. 4A illustrates a phase modulator included in an intensitymodulator. The illustrated intensity modulator is a mach-zehnderinterferometer.

FIG. 4B is a top-view of a phase modulator configured to modulate thephase of a light signal traveling along a branch of an intensitymodulator.

FIG. 5A is a top-view of a phase modulator that includes a plurality ofsub-modulators that are spaced apart from one another. Thesub-modulators are all associated with one pair of transmission lines.

FIG. 5B is a cross section of the modulator of FIG. 5A taken along aline extending between the brackets labeled B in FIG. 5A.

FIG. 6 includes a top-view of an intensity modulator that includes aplurality of phase modulators.

FIG. 7A through FIG. 7E illustrate a method of forming an optical devicethat includes a phase modulator.

DESCRIPTION

A phase modulator is disclosed for the modulation of a light signaltraveling through a waveguide. The phase modulator includes an n-typedoped region and a p-type doped region positioned such that a depletionregion is formed in the waveguide when a bias is not applied to themodulator. The modulator is tuned by applying a reverse bias to themodulator. The reverse bias changes the size of the depletion region.The depletion region has a different index of refraction than thesurrounding light transmitting region. As a result, changing the size ofthe depletion region changes the speed at which the light signal travelsthrough the waveguide. Accordingly, the speed of the light signalthrough the waveguide can be tuned by tuning the bias level applied tothe modulator. Tuning the speed at which the light signal travelsthrough the waveguide allows the modulator to be employed as a phasemodulator.

Tuning the phase modulator by changing the size of the depletion regiondoes not require carrier recombination. When the waveguide is a siliconwaveguide, carrier recombination can be on the order of 10⁴ times slowerthan changes in the depletion region. Accordingly, tuning of thedepletion region provides a faster method of tuning than is availablewith modulators that require carrier recombination. Additionally, theuse of a silicon waveguide provides a platform that can employfabrication techniques that are commonly employed with integratedcircuit manufacturing. As a result, the modulator is both practical andcapable of high-speed modulation.

The phase modulator can be used to provide the modulation in anintensity modulator. For instance, the phase modulator can be configuredto modulate the phase of a light signal traveling along a branch of amach-zehnder interferometer. The increased modulation speed provided bythe phase modulator increases the speed of the intensity modulator.

The dopant concentration in the n-type doped region and in the p-typedoped region affects the performance of the intensity modulator. Forinstance, increasing the dopant concentration can cause undesirably highoptical loss while decreasing the dopant concentration can require anundesirably long modulator to achieve the desired modulation level. Theinventors have found that the optical loss and required length is astrong function of the dopant concentration. As a result, a narrow rangeof dopant concentrations provides the required balance between modulatorlength and optical loss. For instance, when the phase modulator ispositioned on a branch of a Mach-Zehnder interferometer and the lighttransmitting medium is silicon, a suitable concentration for the dopantin the n-type region and/or the p-type region is about 10¹⁶/cm³ to10¹⁷/cm³.

In some instances, the phase modulator includes multiple sub-modulatorsspaced apart from one another along the length of a waveguide. Eachsub-modulator is configured to modulate the light signal as it travelsalong the waveguide. The sub-modulators are each associated with thesame transmission lines that carry the bias signal along the length ofthe phase modulator. The phase modulator is configured such that thebias signal travels through the sub-modulators slower than the lightsignal travels through the waveguide but travels between thesub-modulators quicker than the light signal travels through thewaveguide. The length of the sub-modulators and the spacing between thesub-modulators is selected such that synchronicity between the biassignal and the light signal is substantially retained as the lightsignal and the bias signal travel through the phase modulator.

In some instance, an intensity modulator includes a plurality of thephase modulators. The use of multiple phase modulators can increase theefficiency of the modulators. For instance, the electrical signalapplied to a phase modulator loses energy as it propagates throughtransmission lines in the phase modulator. The loss in the signal energyreduces the efficiency of a single long phase modulator. When thesingle-phase modulator is replaced with a plurality of shorter phasemodulators, the energy loss that occurs in each of the phase modulatorsis reduced. As a result, a more efficient result can be achieved with aplurality of smaller phase modulators. For instance, the summed lengthof the smaller phase modulators can be less than the total length of asingle-phase modulator to achieve the same degree of modulation at thesame bias level. Additionally or alternately, the shorter modulators canemploy a lower bias level to achieve the same degree of modulation thatis achieved in the single longer modulator.

FIG. 1A and FIG. 1B illustrate an optical device. FIG. 1A is a top-viewof the optical device. FIG. 1B is a cross section of the optical deviceshown in FIG. 1B taken along the line labeled B in FIG. 1A. The deviceincludes a light-transmitting medium 10 positioned on a base 12. Asuitable light-transmitting medium 10 includes, but is not limited to,silicon. Recesses 14 are formed in the light-transmitting medium 10 soas to define a ridge 16 extending from a slab 17 of thelight-transmitting medium 10. The ridge 16 defines a waveguide wherelight signals are constrained as they travel through the optical device.The portion of the base 12 adjacent to the light-transmitting medium 10constrains the light signals to a light signal-carrying region withinthe waveguide. For instance, the upper surface of the base 12 can havean index of refraction less than the index of refraction of thelight-transmitting medium 10. The reduced index of refraction reflectslight signals from the light-transmitting medium 10 back into thelight-transmitting medium 10.

The base 12 illustrated in FIG. 1B includes an insulator 18 positionedover a substrate 20. When the light-transmitting medium 10 is silicon, asuitable insulator 18 includes, but is not limited to, silica and asuitable substrate 20 includes a silicon substrate. Asilicon-on-insulator wafer is a suitable platform for an optical devicehaving a silicon light-transmitting medium 10 positioned over a base 12having a silica insulator 18 and a silicon substrate 20.

A filler 22 such as a solid or a gas is positioned in the recesses 14.The filler 22 has an index of refraction lower than the index ofrefraction of the light-transmitting medium 10 in order to constrain thelight signals within the ridge 16. The filler can also provideelectrical isolation between different regions of the optical device.For instance, the filler can provide electrical isolation between thefirst doped region and the second doped region, which are discussed inmore detail below. When the light-transmitting medium 10 is silicon, asuitable filler 22 includes, but is not limited to, silica. A vacuum canalso serve as a suitable filler 22.

An insulating layer 24 is positioned on the light-transmitting medium 10and the filler 22. The insulating layer is illustrated in FIG. 1B but isnot illustrated in FIG. 1A to simplify the illustration. The insulatinglayer 24 can provide electrical insulation and/or optical confinement.When the light-transmitting medium 10 is silicon, a suitable insulatinglayer 24 includes, but is not limited to, low K dielectrics such assilica, and/or silicon nitride. In one example, the insulating layer 24includes a silicon nitride and oxide bi-layer over silicon.

An upper layer is positioned on the insulating layer 24. The upper layeris illustrated in FIG. 1B but is not illustrated in FIG. 1A to simplifythe illustration. The upper layer can serve to reduce or preventcapacitive coupling between different components in the device. Forinstance, the upper layer can prevent or reduce capacitive couplingbetween a first conducting member 26 and a second conducting member 28that are disclosed in more detail below. When the light-transmittingmedium 10 is silicon, a suitable upper layer includes, but is notlimited to, low K dielectrics such as silica.

The phase modulator includes a first conducting member 26 and a secondconducting member 28 as is evident in both FIG. 1A and FIG. 1B. In FIG.1A, the first conducting member 26 and the second conducting member 28are illustrated by dashed lines and are shown as transparent to permit aview of the underlying features. The first conducting member 26 and thesecond conducting member 28 can serve as electrodes but more preferablyserve as transmission lines. Suitable materials for the first conductingmember 26 include, but are not limited to, aluminum, copper and/or theiralloys. Suitable materials for the second conducting member 28 include,but are not limited to, aluminum, copper and/or their alloys.

A first electrical connector 30 provides electrical communicationbetween the first member 26 and a contact portion of the slab 17 locatedadjacent to the waveguide and spaced apart from the waveguide. Secondelectrical connectors 32 provide electrical communication betweencontacts 34 at the top of the ridge 16 and the second member 28. Thefirst electrical connector 30 and the second electrical connectors 32are illustrated in FIG. 1B but are not illustrated in FIG. 1A tosimplify the illustration. The first electrical connectors, the secondelectrical connectors and the contacts provide electrical connectionsbetween electronics and the optics. Suitable materials for the firstelectrical connector 30 include, but are not limited to, tungsten,aluminum, copper and/or their alloys. Suitable materials for the secondelectrical connector 32 include, but are not limited to, tungsten,aluminum, copper and/or their alloys. Suitable materials for thecontacts 34 include, but are not limited to, Al—Si alloys, Ti silicide,and Co silicide.

In some instances, the contacts 34 are a doped non-metal such as dopedsilicon or doped polysilicon. Doped polysilicon can provide the requiredelectrical conduction but can have about two orders of magnitude fewercarriers than the metal. Because increased carrier content is associatedwith increased light absorption, contacts 34 constructed from dopedsilicon can be associated with reduced levels of optical loss relativeto metals. As a result, contacts 34 constructed of doped silicon orpolysilicon may be desired when low levels of optical loss are desired.When the contacts 34 are made of polysilicon, a suitable concentrationof the dopant includes, but is not limited to, concentrations of about10¹⁸/cm³ to 2×10²¹/cm³ or 10¹⁹/cm³ to 2×10²/cm³.

The light-transmitting medium 10 is doped so as to have a first dopedregion 36 and a second doped region 38. When the first doped region 36is an n-type region, the second dope region is a p-type region. When thefirst doped region 36 is a p-type region, the second dope region is ann-type region. In some instances, the first doped region is preferablyan n-type region and the second doped region is preferably a p-typeregion. For instance, certain fabrication techniques may permit easierformation of a p-type region deeper in the light transmitting mediumthat an n-type region. When the contacts 34 are formed of a dopednon-metal, the non-metal is doped with the same type of dopant as thefirst doped region 36 but can be at a higher dopant concentration thanthe first doped region 36.

The first doped region 36 and the second doped region 38 are positionedsufficiently close to one another that a depletion region 40 formsbetween the n-type region and the p-type region when a bias is notapplied to the phase modulator. For instance, FIG. 1B illustrates then-type region in contact with the p-type region. Contact between then-type region and the p-type region may not be necessary although it canincrease the efficiency of the modulator. The resulting interface issubstantially parallel to the top of the ridge 16 and/or the base 12 andis positioned in the ridge 16.

The depletion region 40 results from a migration of carriers between then-type region and the p-type region until a potential forms thatprevents additional migration. This migration results in a lack ofcarriers in the depletion region. For instance, the depletion region 40has a carrier concentration of less than about 1×10¹⁵/cm³. The n-typeregion and a p-type region are positioned so the depletion region 40 ispositioned in the light signal-carrying region of the waveguide. Forinstance, FIG. 1C illustrates the depletion region 40 that forms fromthe doped region configuration illustrated in FIG. 1B. A suitableconcentration of carriers in the p-type region includes values greaterthan 1×10¹⁵/cm³, 1×10¹⁶/cm³, 3.5×10¹⁶/cm³, or 5.0×10¹⁷/cm³. A suitablevalue for the concentration of carriers in the n-type region includesvalues greater than 1×10¹⁵/cm³, 2×10¹⁶, 5×10¹⁶, and 1×10¹⁸ cm⁻³.

A secondary doped region 44 is formed at the contact portion of the slab17. The secondary doped region 44 can contact the adjacent doped regionand can include the same type of dopant as the adjacent doped region.For instance, in FIG. 1B, the underlying doped region is the seconddoped region 38. Accordingly, when the phase modulator is constructed asillustrated in FIG. 1B, the secondary doped region 44 can contact thesecond doped region and has a dopant type that is the same as the seconddoped region 38. The secondary doped region 44 can have a higher dopantconcentration than the adjacent doped region. For instance, the dopantconcentration in the secondary doped region 44 can be more than 10 timesthe dopant concentration in the adjacent doped region or more than 1000times the dopant concentration in the adjacent doped region. Theelevated dopant concentration reduces the contact resistance of thephase modulator and accordingly provides an increased modulation speed.Suitable concentrations for the dopant in the secondary doped region 44include, but are not limited to, concentrations greater than 1×10¹⁸/cm³,1×10¹⁹/cm³, 5×10¹⁹/cm³, 1×10²⁰/cm³. Increasing the dopant concentrationcan increase the amount of optical loss. As a result, the secondarydoped region 44 is positioned remote from the light signal-carryingregion in order to reduce optical loss resulting from the increaseddopant concentration. For instance, the secondary doped region 44 ispositioned on a portion of the slab 17 adjacent to the recess 14. Thislocation can reduce interaction between a light signal in the waveguideand the secondary doped region 44. In some instances, the secondarydoped region 44 can be positioned in the recess 14 or in the bottom ofthe recess 14.

The first member 26 and the second member 28 are connected toelectronics (not shown) that can apply a bias between the firstconducting member 26 and the second conducting member 28. Accordingly, abias is formed between the top of the ridge 16 and the contact portionof the slab 17. The bias can be a reverse bias. Changing the level ofbias changes the size and/or shape of the depletion region. Forinstance, increasing the reverse bias can increase the size of thedepletion region. As an example, FIG. 1D illustrates the depletionregion of FIG. 1C after an increased reverse bias has been applied tothe phase modulator. FIG. 1B, FIG. 1C and FIG. 1D illustrate the firstdoped region and the second doped region occupying the entire lightsignal carrying region. This arrangement can provide an increasedpotential tuning efficiency.

The depletion region 40 has a different index of refraction than thelight transmitting region located adjacent to the depletion region. Forinstance, when the light-transmitting medium 10 is silicon, thedepletion region 40 has a higher index of refraction than that of thesurrounding silicon. As a result, the depletion region 40 slows thelight signal as the light signal travels through the depletion region.As a result, increasing the size of the depletion region 40 furtherslows the speed at which the light signal travels through the waveguide.Accordingly, the speed of the light signal through the waveguide can betuned by tuning the bias level. Additionally, because this phase tuningis based on tuning of the depletion region, tuning of the phasemodulator does not involve carrier re-combination. Carrier recombinationis on the order of 1000 times slower than changes in the depletionregion. Accordingly, the phase modulator can be on the order of 1000 to10000 times faster than phase modulators that require carrierrecombination.

A forward bias can be applied to the phase modulator. The forward biaswill shrink the size of the depletion region. Accordingly, when thelight-transmitting medium 10 is silicon, increasing the forward bias canaccelerate the light signal. However, once the forward bias rises abovea threshold, the forward bias can result in current flow that requiresrecombination as the forward bias drops toward the threshold. Becausetuning that requires recombination is slower than tuning of thedepletion region, it may not be desirable to use the forward bias abovelevels where significant current flow occurs.

The concentration of the dopants in the doped regions influences theperformance of the phase modulator. For instance, the dopants can causelight absorption. As a result, increasing the dopant level can causeundesirably high levels of optical loss. Decreasing the dopant level canreduce the tuning efficiency by requiring a higher bias level to achievethe same level of phase modulation. As a result, when the dopant levelis reduced, the length of the phase modulator must be increased toprovide the desired level of phase modulation for a give bias level.Suitable dopants for the n-type region include, but are not limited to,phosphorus and/or arsenic. Suitable dopants for the p-type regionsinclude, but are not limited to, boron.

Although FIG. 1B illustrates the interface between the first dopedregion 36 and the second doped region 38 as being positioned in theridge 16, first doped region 36 and the second doped region 38 can beconstructed so the interface is below the ridge 16. In these instances,the doped region in the ridge 16 and the secondary doped region 44 maybe the same type of doped region. For instance, the doped region in theridge 16 and the secondary doped region 44 may both be an n-type regionor they may both be a p-type region.

The waveguide can be dimensioned so as to be a single-mode waveguide ora multi-mode waveguide. An example of a single mode waveguide has aridge 16 with a height of about 0.55 μm, a distance of about 0.5 μm fromthe top of the ridge 16 to the interface between the first doped region36 and the second doped region 38, a height of about 1.4 μm from the topof the ridge 16 to the base, and a top of the ridge 16 width of about1.4 μm.

The phase modulator construction of FIG. 1A and FIG. 1B shows thecontacts 34 at the corners of the ridge 16 and spaced apart. Forinstance, the contacts 34 do not extend all the way across the top ofthe ridge 16. FIG. 2A shows the profile of the fundamental mode of alight signal traveling along the waveguide. The position of the contacts34 reduces the interaction between the contacts 34 and the light signal.For instance, positioning the contacts 34 at the edge of the ridge 16provides a larger distance between the profile and the contacts 34 thanwould occur if the contacts 34 were positioned over the center of theridge 16. As a result, the positioning of the contacts 34 can reduce theoptical loss resulting from absorption by the contacts 34. As notedabove, the contacts 34 can also be constructed of a doped non-metal inorder to further reduce the light absorption.

Although two contacts 34 are illustrated, can include a single one ofthe contacts 34 as illustrated in FIG. 2B. The use of the single contact34 can further reduce the amount of optical loss associated with thecontact 34. Although the contact 34 is shown positioned on the corner ofthe ridge 16 that is furthest from the secondary doped region 44, thecontact 34 can be positioned on the corner of the ridge 16 that iscloses to the secondary doped region 44.

FIG. 2C is a cross section of the optical device showing an alternateconstruction of the interface between the waveguide and the contacts 34.Conductors 48 are positioned on top of the ridge 16 and are spaced apartfrom one another. Spacers 50 are positioned between the conductors 48.The spacers 50 can have an index of refraction that is less than theindex of refraction of the light-transmitting medium 10. The reducedindex of refraction effectively pushes the fundamental mode into thewaveguide and away from the contact 34. The push of the fundamental modeaway from the contact 34 reduces the amount of light absorbed by thecontact 34. As a result, the contact 34 can be a metal without the highabsorption levels associated with metal contacts 34. As noted above, thecontact 34 can be a doped non-metal. Suitable materials for theconductors 48 include, doped silicon. When the light-transmitting medium10 is silicon, a suitable material for the conductors includes, silicon,polysilicon, doped silicon and doped polysilicon.

When the interface between the waveguide and the contact(s) 34 isconstructed in accordance with FIG. 2C, increasing the portion of theridge 16 top width that contacts a conductor can reduce the resistanceof the interface. Reducing this resistance can increase the speed of thephase modulator.

Although the embodiment of the phase modulator illustrated in FIG. 1Aand FIG. 1B is configured to apply a potential difference between thetop of the ridge 16 and a location adjacent to the ridge 16. The phasemodulator can be configured such that the potential is formed betweenopposing sides of the ridge 16. For instance, FIG. 3A illustrates anembodiment of the phase modulator where contact regions are positionedon opposing sides of the ridge 16. For instance, the secondary dopedregions 44 are positioned on opposing sides of the ridge 16.Accordingly, the phase modulator is operated by applying a bias betweenopposing sides of the ridge 16. The first doped region 36 and the seconddoped region 38 are formed so a depletion region 40 is formed in, theridge 16. For instance, the first doped region 36 and the second dopedregion 38 are formed so as to have a substantially vertical interfacepositioned within the ridge 16.

In some instances, the phase modulator embodiment of FIG. 1A provideshigher speed performance relative to the embodiment of FIG. 3A. Thepositioning of both contact regions on opposing sides of the ridge 16can increase the distance between the contact regions and canaccordingly increase the resistance associated with the phase modulator.This increase in resistance can slow the performance of the phasemodulator.

The phase modulator can be adapted to other waveguide types. Forinstance, FIG. 3B illustrates the phase modulator employed inconjunction with a channel waveguide. The first doped region 36 and thesecond doped region 38 are formed so a depletion region 40 is formed inthe waveguide. For instance, the first doped region 36 and the seconddoped region 38 are formed so as to have a substantially verticalinterface positioned within the waveguide. The resulting depletionregion 40 is also positioned in the waveguide. The contacts 34 arepositioned at the upper edges of the waveguide. As a result, the phasemodulator is operated by applying a bias between the upper edges of thewaveguide.

FIG. 4 illustrates above phase modulator employed in a mach-zehnderinterferometer to provide a high-speed intensity modulator. Theintensity modulator includes an input waveguide 52, an output waveguide54, a first branch 56, and second branch 58. The phase modulator 60 ispositioned along the second branch 58. During operation of the intensitymodulator, a light signal travels along the input waveguide 52. A firstportion of the light signal enters the first branch 56 and a secondportion of the light signal enters the second branch 58. The firstportion of the light signal and the second portion of the light signalthen re-combine at the output waveguide 54. The phase modulator isoperated so as to tune the phase of the second portion of the lightsignal relative to the first portion of the light signal. When the firstportion of the light signal and the second portion of the light signalenter the output waveguide 54 in phase, there is high intensity outputfrom the intensity modulator. When the first portion of the light signaland the second portion of the light signal are out of phase by π, thereis low intensity output from the intensity modulator. Accordingly, theintensity modulator can be switched between the low intensity output andthe high intensity output by operating the phase modulator to create a πphase difference or zero phase difference.

In some instances, it is desirable for an intensity modulator such as aMach-Zehnder interferometer to provide intensity modulation on the orderof 10 to 40 Gbit/s with low levels of optical loss. Accordingly, thehigh-speed features of the phase modulator can be important when thephase modulator is employed for intensity modulation. Additionally, thelow optical loss features of the phase modulator can also becomedesirable when the phase modulator is employed for intensity modulation.

FIG. 4B is a top-view of a phase modulator constructed according to FIG.1A employed in conjunction with a branch waveguide of a Mach-Zehnderinterferometer. In FIG. 4B the first conducting member 26 and the secondconducting member 28 are illustrated by dashed lines and are shown astransparent to permit a view of the underlying features. Additionally,the first electrical connector, the second electrical connectors, theinsulating layer, and upper layer are not illustrated in FIG. 4B tosimplify the illustration.

The concentration of the dopants in the first doped region 36 and thesecond doped region 38 of the phase modulator influences the performanceof the intensity modulator. For instance, the dopants can absorb thelight signal. As a result, increasing the dopant level can causeundesirably high levels of loss. Decreasing the dopant level can reducethe tuning efficiency. As a result, the length of the phase modulatormust be increased to provide the desired level of tuning. For instance,it may be desirable for the phase modulator to provide a π phase shift.Increasing the phase shift to this magnitude can require a phasemodulator with an increased length. As a result, decreasing the dopantconcentration can result in the phase modulator being impractically longto be effective in a Mach-Zehnder interferometer. Accordingly, when thephase modulator is included in an intensity modulator, the dopantconcentration in the first phase and the second phase must be chosen toachieve a balance achieved between the length of the phase modulator andthe optical loss caused. When the phase modulator is positioned on abranch of a Mach-Zehnder interferometer and the light-transmittingmedium 10 is silicon, a suitable concentration for the dopant in then-type region about 10¹⁵/cm³ to 10¹⁸/cm³ and more preferably 10¹⁶/cm³ to10¹⁷/cm³. When the phase modulator is positioned on a branch of aMach-Zehnder interferometer and the light-transmitting medium 10 issilicon, a suitable concentration for the dopant in the p-type regionabout 10¹⁵/cm³ to 10¹⁸/cm³ and more preferably 10¹⁶/cm³ to 10¹⁷/cm³.

Electronics 64 are configured to apply an electrical signal to the firstmember 26 and the second member 28. The electrical signal establishesthe desired bias between the first member 26 and the second member 28.The electronics are configured to apply the electrical signal to one endof the first member 26 and the second member 28. As a result, the firstmember 26 and the second member 28 act as transmission lines that carrythe electrical signal along the length of the first member 26 and thesecond member 28. For instance, the arrow labeled E in FIG. 4Billustrates the direction which the electrical signal propagates throughthe second member 28. The arrow labeled L illustrates the directionwhich the light signal propagates through the waveguide. As is evidentin FIG. 4B, the light signal and the electrical signal travelsubstantially parallel to one another. The light signal and theelectrical signal can travel at about the same speed. As a result, theelectrical signal continues to operate on the same portion of the lightsignal as they both travel through the phase modulator. In someinstances, similar propagation speeds between the light signal and theelectrical signal can be achieved by changing the dimensions of thefirst member 26 and/or the dimensions of the second member 28 toinfluence the propagation speed of the electrical signal.

The phase modulator can include a plurality of sub-modulators 66 asillustrated in FIG. 5A and FIG. 5B. FIG. 5A is a top-view of the phasemodulator and FIG. 5B is a cross section of the phase modulator takenalong the longitudinal axis of the waveguide in FIG. 5A. For instance,FIG. 5B is a cross section of the phase modulator taken along a lineextending between the brackets labeled B in FIG. 5A. In FIG. 5A thefirst conducting member 26 and the second conducting member 28 areillustrated by dashed lines and are shown as transparent to permit aview of the underlying features. Additionally, the first electricalconnector 30, and the second electrical connectors 32 are notillustrated in FIG. 5A to simplify the illustration. The repeat diagonallines in FIG. 5A illustrate the locations of undoped light-transmittingmedium 10.

The illustrated portion of the phase modulator includes foursub-modulators 66 that are each associated with a first member 26 and asecond member 28. Each sub-modulator 66 includes a different pair ofdoped regions. For instance, each sub-modulator 66 includes a firstdoped region 36 and a second doped region 38. Accordingly, an electricalsignal placed on the first member 26 and the second member 28 canprovide modulation of a light signal at each of the sub-modulators 66.

The electrical signal propagates through the first member 26 asillustrated by the arrow labeled E. The dashed portions of the arrowindicate when the electrical signal is passing through a sub-modulator66 and the unbroken portions of the arrow indicate when the electricalsignal passing between sub-modulators 66. The electrical signal travelsfaster when it is traveling between sub-modulators 66 than when thelight signal is traveling through a sub-modulator 66. As a result, thelength of the sub-modulators 66 and the separation between thesub-modulators 66 can be selected to control the time required for theelectrical signal to propagate the length of the first member 26 and thesecond member 28.

It can become necessary to control the time for an electrical signal topropagate through the phase modulator as the length of the modulatorincreases. For a give bias between the first member 26 and the secondmember 28, the time for which the light signal is exposed to theelectrical signal must be increased in order to increase the phasedifferential caused by the phase modulator. In order to achieve a πphase differential, the first member 26 and the second member 28 oftenmust have a length on the order of several centimeters in length. Thislength can emphasize the difference between the propagation speed of thelight signal and the electrical signal. The sub-modulators 66 can bearranged to reduce the effects of the speed differential. For instance,the electrical signal travels through the first member 26 faster thanthe light signal travels through the waveguide when the electricalsignal is between sub-modulators 66, the light-transmitting medium 10 issilicon, and the first member 26 and the second member 28 are aluminum.However, under these same conditions, the electrical signal travelsthrough the first member 26 more slowly than the light signal travelsthrough the waveguide when the electrical signal is traveling through asub-modulator 66.

Under the conditions described above, the sub-modulators 66 can beconfigured to passively preserve synchronize between the electricalsignal and the light signal as these signals travel through the phasemodulator. For instance, the sub-modulators 66 can be configured suchthat the electrical signal and the light signal are synchronized whenthey enter the first sub-modulator 66 but that the electrical signalbegins to lag the light signal as they travel through the sub-modulator66. The spacing between the sub-modulators 66 can then be selected sothe electrical signal catches back up to the light signal or passes thelight signal before both signals enter the next sub-modulator 66. As aresult, the length and spacing of the sub-modulators can be selectedsuch that the average speed of the light signal through the waveguideand the average speed of the electrical signal through the transmissionline are about the same. As an example, the length and spacing of thesub-modulators can be selected such that the average speed of the lightsignal through the phase modulator is equal to the average speed of theelectrical signal through the phase modulator +/−0.1%, +/−1%, +/−10%, or+/−40%. As a result, the bias signal modulates substantially the sameportion of the light signal as the light signal and the bias signaltravel through the phase modulator.

The number, configuration, and spacing of the sub-modulators 66 in thephase modulator can be selected to control the degree of synchronicitybetween the electrical signal and the light signal. Reducing the lengthof the sub-modulators 66 can increase the number of sub-modulators 66that are needed to achieve a particular degree of modulation. However,shorter sub-modulators 66 reduce the lag between the electrical signaland the light signal. Additionally, less distance is required betweenshorter sub-modulators 66 in order to return synchronicity between thelight signal and the electrical signal. As a result, reducing thesub-modulator 66 length can increase the synchronicity between theelectrical signal and the light signal.

In some instances, the dimensions of the first member and the secondmember can also be changed to influence the speed of the electricalsignal through the phase modulator. For instance, the thickness of thefirst member and/or the second member can be changed to tune thepropagation speed of the electrical signal through the phase modulator.

A suitable sub modulator spacing ratio (average length of the submodulators: the average spacing between the sub-modulators 66) is in arange of 100:1 to 1:100, 1:10 or 10:1, 0.2:1 to 2:1. When thelight-transmitting medium 10 is silicon, a preferred sub modulatorspacing ratio is in a range of 100:1 to 1:100.

An intensity modulator can include a plurality of the phase modulatorsas illustrated in FIG. 6. FIG. 6 is a top-view of a portion of anoptical device. The optical device includes a mach-zehnderinterferometer configured to serve as an intensity modulator. Aplurality of the phase modulators 67 are positioned along a branch ofthe interferometer. Each phase modulator can include a singlesub-modulator 66 or a plurality of sub-modulators 66 as illustrated inFIG. 5A. Each phase modulator is in electrical communication with devicecontrol electronics 68. The device control electronics 68 include aplurality of device drivers 70 that are each in electrical communicationwith an associated phase modulator. A controller 72 is in electricalcommunication with the device drivers 70.

During operation of the device, the device drivers 70 apply theelectrical signal to the associated phase modulator. The controller 72is configured to control the timing at which the electrical signal isapplied to each phase modulator. For instance, the device controlelectronics 68 create a delay between when the electrical signal isapplied to each phase modulator so the electrical signals aresynchronized with the light signal traveling through the waveguide. FIG.6 illustrates this synchronization. As a light signal travels throughthe waveguide, the longitudinal location of a particular segment of thelight signal is shown at times labeled L₁, L₂ and L₃. The light signalis modulated by three electrical signals labeled E₁, E₂, and E₃. Thedevice control electronics 68 apply the electrical signal E₁ at the timelabeled L₁, the electrical signal E₂ at the time labeled L₂, and theelectrical signal labeled E₃ at the time labeled L₃. As a result, theelectrical signals labeled E₁, E₂, and E₃ each modulate the same portionof the light signal. Accordingly, the effects of E₁, E₂, and E₃ add upto provide the total modulation for that portion of the light signal.

Additionally, the device control electronics 68 can be configured todetermine the electrical signal that must be applied to each phasemodulator in order to achieve the desired degree of modulation. Forinstance, the electrical signals E₁, E₂, and E₃ can be the same ordifferent. The device control electronics 68 can determine whatcombination of electrical signals electrical signals E₁, E₂, and E₃ willprovide the desired result. For instance, if the intensity modulator isused as a digital modulator and the target portion of the light signalis to show a “1” (as opposed to a zero), the device control electronicscan identify and/or apply the combination of E₁, E₂, and E₃ needed toproduce a “1.”

Suitable device control electronics 68 include, but are not limited to,firmware, hardware and software or a combination thereof. Examples ofsuitable control electronics include, but are not limited to, analogelectrical circuits, digital electrical circuits, processors,microprocessors, digital signal processors (DSPs), computers,microcomputers, ASICs, and discrete electrical components, orcombinations suitable for performing the required control functions. Insome instances, the control electronics includes a memory that includesinstructions to be executed by a processing unit during performance ofthe control and monitoring functions.

The electrical signal applied to a phase modulator loses energy as itpropagates through the first member 26 and the second member 28. Theloss in the signal energy reduces the efficiency of a single long phasemodulator. When the single long phase modulator is replaced with aplurality of shorter phase modulators, the energy loss that occurs ineach of the phase modulators is reduced. As a result, a more efficientresult can be achieved with a plurality of smaller phase modulators. Asuitable length of the first member 26 and the second member 28 is lessthan 1 cm, 1 mm or 100 um. When the modulators are included in anintensity modulator having silicon waveguides, a suitable length of thefirst member 26 and the second member 28 is in a range of 0.1 mm to 1cm. As a result, a suitable length for a modulator included in anintensity modulator having silicon waveguides is in a range of 1 mm to10 cm.

Although the intensity modulator of FIG. 6 is shown with three phasemodulators, the intensity modulator can include one phase modulator, twophase modulators, or four or more phase modulators. Although the devicedrivers 70 are shown as being positioned on the optical device, thedevice drivers 70 can be positioned off the optical device.Additionally, the device drivers 70 need not be independent of thecontroller 72 and can be integrated into the controller 72.

FIG. 7A through FIG. 7E illustrate a method of forming an optical devicethat includes the phase modulator. The method can be performed on awafer having a light-transmitting medium positioned on a base. Anexample of a suitable wafer includes, but is not limited to, asilicon-on-insulator wafer.

The material for the insulating layer 24 is deposited on the wafer. Forinstance, when the insulating layer 24 is to be constructed from a layerof silicon nitride over a layer of silica, a layer of silica can beformed on the wafer followed by formation of a layer of silicon nitride.A suitable method of forming the layer of silica on a layer of siliconincludes, but is not limited to thermal growth. A suitable method offorming the layer of silicon nitride includes, but is not limited to,deposition techniques such a low-pressure chemical vapor deposition(LPCVD).

The insulating layer 24 is patterned such that the location where thewaveguide ridge 16 is to be formed is protected and the recesses 14 areexposed. A suitable method of patterning the insulating layer 24includes, but is not limited to, photolithography and etching. Theresult is then etched so as to generate the device precursor illustratedin FIG. 7A.

The doped regions can be formed before or after formation of theinsulating layer 24 on the device precursor. Suitable methods forforming the doped regions include, but are not limited to, ionimplantation, or diffusion. In some instance, one or more regions of theoptical device are masked during the formation of a doped region(s). Themask can protect the one or more regions from the dopant. Suitable masksfor use during formation of the doped regions include, but are notlimited to photoresist, and oxide. The doped regions are not illustratedin FIG. 7A through FIG. 7E in an effort to simplify the illustrations.

The filler 22 is deposited on the device precursor and the result isplanarized to generate the device precursor illustrated in FIG. 7B. Asuitable method for depositing a filler such as silica includes, but isnot limited to, deposition methods such as sputtering and chemical vapordeposition. A suitable method for planarizing the filler includes, butis not limited to, chemical-mechanical polishing (CMP). During CMP of amaterial such as silica, an insulating layer 24 constructed of amaterial such as silicon nitride can serve as a stop. Accordingly, theinsulating layer 24 remains substantially intact after the planarizationprocess.

Portions of the insulating layer 24 are removed so as to generateopenings 84 for the contacts. A layer of the contact material 80 isdeposited on the result so as to provide the device precursorillustrated in FIG. 7C. A suitable method for removing the insulatinglayer 24 include, but are not limited to, photolithography and dryetching. A suitable method for depositing contact materials 80 such aspolysilicon includes, but is not limited to, deposition methods such aschemical vapor deposition (CVD).

The contact material 80 is patterned so as to form the contacts 34 onthe device precursor. A suitable method of patterning the contactmaterial 80 includes, but is not limited to, photolithography andetching.

A first portion of the upper layer material 82 is deposited on thedevice precursor. The first portion of the upper layer material 82 ispatterned so as to expose openings 84 for the connectors and theconnector material 86 is deposited on the device precursor so theconnector material 86 fills the holes as shown in FIG. 7D. A suitablemethod for depositing an upper layer material such as silica includes,but is not limited to, sputtering, spin coating, and chemical vapordeposition (CVD). A suitable method of patterning the first portion ofthe upper layer includes, but is not limited to, photolithography andetching. A suitable method for depositing a connector material 86 suchas tungsten includes, but is not limited to, chemical vapor deposition(CVD) or sputtering.

The device precursor is planarized to the level of the first portion ofthe upper layer. A suitable method for planarizing the connectormaterial 86 includes, but is not limited to, chemical-mechanicalpolishing (CMP).

A second portion of the upper layer material 88 is deposited on thedevice precursor. The second portion of the upper layer material 88 ispatterned so as to expose openings for the first conducting member 26and the second conducting member 28. The material for the firstconducting member 26 and the second conducting member 28 is deposited onthe device precursor so the connector material 86 overfills the holes.The first member 26 and the second member 28 are formed by planarizingthe device precursor to the second portion of the upper layer material88. A third portion of the upper layer material 90 is deposited on thedevice precursor to generate the optical device of FIG. 7E. A suitablemethod for depositing second upper layer material such as silica andsilicon nitride includes, but is not limited to, sputtering, spincoating, and deposition methods such as chemical vapor deposition (CVD).A suitable method of patterning the second portion of the upper layermaterial includes, but is not limited to, photolithography and etching.A suitable method for depositing a material such as aluminum for thefirst conducting member 26 and the second conducting member 28 includes,but is not limited to, sputtering and chemical vapor deposition (CVD). Asuitable method for planarizing the material for the first conductingmember 26 and the second conducting member 28 includes, but is notlimited to, chemical-mechanical polishing (CMP). A suitable method fordepositing second upper layer material such as silica includes, but isnot limited to, sputtering, spin coating, and deposition methods such aschemical vapor deposition (CVD).

The above method for generating the optical device is disclosed in thecontext of a silicon-on-insulator wafer. Accordingly, the materials andmethods are disclosed in conjunction with a silicon light-transmittingmedium 10. These materials and techniques may be effective when employedwith different light-transmitting media, however, differentlight-transmitting media may require changes to the above techniquesand/or materials.

Although the intensity modulator is disclosed in the context of adigital modulator where a high intensity output or a low intensityoutput is desired, the intensity modulator can also be operated as ananalog modulator. For instance, the phase modulator(s) included in theintensity modulator can be operated so as to produce intensities betweenor outside of the high intensity output and the low intensity output.

Other embodiments, combinations and modifications of this invention willoccur readily to those of ordinary skill in the art in view of theseteachings. Therefore, this invention is to be limited only by thefollowing claims, which include all such embodiments and modificationswhen viewed in conjunction with the above specification and accompanyingdrawings.

1. An optical device, comprising: a phase modulator that includes awaveguide on a substrate, the waveguide having a light signal carryingregion; an n-type region having a proximity to a p-type region thatcauses a depletion region to form when a bias is not applied to themodulator, the depletion region being at least partially positioned inthe light signal carrying region of the waveguide; and the phasemodulator includes a plurality of sub-modulators connected to atransmission line configured to carry a bias signal to each of thesub-modulators, the sub-modulators being spaced apart from one anotheralong a length of the waveguide, the length of the sub-modulators alongthe waveguide and the spacing between the sub-modulators being selectedsuch that the average speed of the bias signal through the transmissionline is about the same as the average speed of a light signal travelingthrough the phase modulator.
 2. The device of claim 1, wherein then-type region contacts the p-type region.
 3. The device of claim 1,further comprising: electronics configured to apply a reverse bias tothe modulator so as to tune the size of the depletion region.
 4. Thedevice of claim 1, wherein the depletion region is at least partiallypositioned in a ridge of the waveguide.
 5. The device of claim 1,further comprising: electrical contacts positioned on a ridge of thewaveguide such that the electrical contacts are spaced apart from oneanother at the top of the ridge.
 6. The device of claim 5, wherein theelectrical contacts include doped polysilicon.
 7. The device of claim 1,further comprising: electrical connections for applying a bias signal tothe waveguide at locations that are spaced apart from one another on topof the waveguide.
 8. The device of claim 7, wherein the electricalconnections are configured to apply the bias signal to positions onopposing edges of the ridge.
 9. The device of claim 1, furthercomprising: electrical connections for applying a bias signal at acontact region of the modulator located adjacent to the waveguide andspaced apart from the waveguide.
 10. The device of claim 9, wherein thecontact region of the modulator includes a secondary doped region incontact with and having a higher carrier concentration than the n-typeregion or the p-type region.
 11. The device of claim 10, wherein thesecondary doped region includes the same dopant type as the n-typeregion or the p-type region with which the secondary doped region is incontact.
 12. The device of claim 1, wherein the waveguide is defined ina light transmitting medium that includes recesses that define a ridgeof the waveguide, and further comprising: electrical connections forapplying a bias signal to the light transmitting medium at a contactlocation that is fully or partially on an opposite side of a recess fromthe waveguide.
 13. The device of claim 1, wherein the waveguide issilicon and a concentration of the carriers in the n-type region is10¹⁶/cm³ to 10¹⁷/cm³.
 14. The device of claim 1, wherein the waveguideis silicon and a concentration of the carriers in the p-type region is10¹⁶/cm³ to 10¹⁷/cm³.
 15. The device of claim 1, wherein thetransmission line is configured such that a direction of propagation ofthe bias signal through the transmission line is substantially parallelto the waveguide.
 16. The device of claim 1, wherein the waveguide issilicon and a concentration of the carriers in the n-type region is10¹⁶/cm³ to 10¹⁷/cm³ and a concentration of the carriers in the p-typeregion is 10¹⁶/cm³ to 10¹⁷/cm³; and electrical connections for applyinga bias between a top of the ridge waveguide and a location adjacent tothe ridge waveguide; and electronics for applying a reverse bias betweenthe top of the ridge waveguide and the location adjacent to the ridgewaveguide.
 17. The device of claim 1, wherein the phase modulatorpositioned along a branch waveguide of a mach-zehnder interferometer.18. The device of claim 17, wherein the phase modulator is one of aplurality of phase modulators positioned along the branch of themach-zehnder interferometer.
 19. The device of claim 1, wherein thetransmission line consists of a continuous and unbroken material. 20.The device of claim 1, wherein a second transmission line is configuredto carry the bias signal from each of the sub-modulators such that thesub-modulators are connected in parallel between the transmission lineand the second transmission line.